Modern TVs do not power on with the application of power, unlike the tube TV's of yesteryear. This is a consumer convenience so that your TV does not turn on, for example, after power is restored from an outage in your home or office. You can usually only control the TV power via IR control.
In the case of Signage, there is another consideration. After you do turn on the TV, it can take a few moments before the network connection is active, especially if you are using WiFi. So if you quickly turn on the TV, navigate to the Signage App, and start it, it could fail because there is no network connection yet. Wait a moment and try again and it works because the network is now active.
There are several possible solutions to automation:
Use BluRay Player and leave it on
The Signage App works the same on Samsung BluRay players (under $100). While you may be motivated to turn off a large screen TV at the end of the day, you might elect to keep the BluRay player running. When you turn on the power to the TV, it can default to the last used HDMI input, in this case from the BluRay player.
The Discover Video SoftTopBox is a palm sided computer that is used with Digital Signage for any display (that accepts HDMI). While more expensive, it can also be the controller for an optional IR remote control device that can be used to turn the TV power on/off from the DEVOS system.
Manually Tun On TVs and Navigate To The App
In many cases, simply using the TV's Remote Control every morning and evening is not a burden. In the morning, turn on the TV and start the Signage app. In the evening, turn of the TV.